Person A and Person B have argument. People on Person A's friends list stand up for Person A, people on Person B's friends list stand up for Person B. The person they stand up for couldn't possibly be wrong, and everything is taken at face value.

I'm sure its like that in real life, too, its just not easily documented and seen.

My new year's resolution, coming a few months late, is to not defend anyone in these situations, just to be supportive. That, and not think I really know someone until I've spent time with them, face to face.